The present invention relates to an ICR ion trap comprising electrically conductive side plates of equal axial length extending in parallel to one axis, and electrically conductive end plates extending perpendicularly to the said axis, closing the space defined by the said side plates and being electrically insulated from the latter, and a voltage source serving for applying trapping potentials to the side plates and end plates.
Ion traps of this kind have been used in ICR mass spectrometers and serve the purpose of trapping the ions of substances intended to be examined by mass spectroscopy, using the cyclotron resonance. For trapping negative ions, the end plates are in this case maintained at a negative potential, relative to the side plates, while for trapping positive ions the potential of the end plates must be positive relative to that of the side plates.
From the above it appears that with the known ICR ion traps the polarity of the potential of the end plates, relative to the side plates, determines the polarity of those ions that can be trapped by means of such an ion trap. If, as is usually the case, the ions are generated inside the ion trap by exposure of the substance to be examined to radiation, for example by application of a laser beam or an electron beam, then negative and positive ions may occur at the same time, in particular when an electron beam is applied, and of the two types of ions so obtained one will always be lost although it may absolutely be of interest to examine both types of ions. On the other hand, it may also be of interest to examine, by means of mass spectroscopy, any recombination reactions between positive and negative ions, but this the known ICR ion traps generally do not allow. Consequently, there exists a demand for ion traps which would permit to trap both positive and negative ions at the same time.